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RP-bit set - Multicast

mac_mac_net83
Level 1
Level 1

Hi All, 

 

I have a question about SPT switchover and the role of the RP-bit set. 

As far as I understand, once the SPT switchover is completed, the last hop router will send a prune message back to the shared tree. 

 

On the upstream router where the last hop router is connected , there will be a "R" flag set and at the same time a"P" flag to tell the other routers upstream to prune traffic on the least optimized path , thus all OILs will become NULL. 

 

However in my case it is different. I have example a pri path (where route to the RP is more preferred)  and secondary path (where the route to the source is more preferred). I can verify on the secondary path (where the source of the multicast traffic is more preferred), that the S,G entries are set to T. Meaning there is a successful switchover. 

 

However, what I see now on the routers on the primary path , that they only have the (S,G) entries flagged as "R" and not "RP".

In this case the OIL is still active (not null) and pointing to the last hop router or receiver direction , which I beleive that MC traffic is flowing on both directions now....If this is live in the network, this could mean duplicate MC traffic ?

 

I still expect the last hop router to send a prune message , but I am not receiving it....

What could be the reasons?

 

mac

2 Accepted Solutions

Accepted Solutions

Hi @mac_mac_net83,

Those outputs show that Routing Table on the LHR (R8#) is pointing to R7# to reach both, the RP (11.11.11.11) and the Source of the Multicast traffic (192.168.1.2).

R8#show ip mroute 234.2.2.2

(*, 234.2.2.2), 00:14:55/stopped, RP 11.11.11.11, flags: SJCL
 Incoming interface: FastEthernet0/0, RPF nbr 78.78.78.7
Outgoing interface list:
 FastEthernet1/0, Forward/Sparse, 00:12:49/00:02:17
 Loopback8, Forward/Sparse, 00:14:51/00:00:08


(192.168.1.2, 234.2.2.2), 00:11:28/00:01:18, flags: LJT
 Incoming interface: FastEthernet0/0, RPF nbr 78.78.78.7
Outgoing interface list:
 Loopback8, Forward/Sparse, 00:11:28/00:00:31
 FastEthernet1/0, Forward/Sparse, 00:11:28/00:02:17

Therefore R8# is not sending PIM Prune with RP-bit set to R6#.

Use show ip rpf 11.11.11.11 and show ip route 11.11.11.11 to know the reason.

 

Cheers.

View solution in original post

Hello Mark,

as noted by Hector

from the point of view of the last hop router the upstream interface for the RP address 11.11.11.11 and for the source 192.168.1.2 is the same

(*, 234.2.2.2), 00:14:55/stopped, RP 11.11.11.11, flags: SJCL
>>Incoming interface: FastEthernet0/0, RPF nbr 78.78.78.7

(192.168.1.2, 234.2.2.2), 00:11:28/00:01:18, flags: LJT
>>>Incoming interface: FastEthernet0/0, RPF nbr 78.78.78.7

 

so R8 has no need to send an RP bit set PIM Prune message to anyone.

 

R8 has the same RPF neighbor 78.78.78.7 for both so it is attached to likely R7.

 

check with

show ip rpf 11.11.11.11

show ip route 11.11.11.11

 

show ip route 192.168.1.2

show ip rpf 192.168.1.2

 

Edit:

depending on topology R7 can have won a competition with R6 with a PIM assert message for both the RP address and the source address.

Or R7 is providing the best path to both the RP address and the source address.

Or R8 -R6 PIM neighborship is not up in their  dedicated link.

 

Hope to help

Giuseppe

 

 

View solution in original post

7 Replies 7

Hello,

 

in order to give you a meaningful answer, we probably need to lab your setup. Can you provide configs and topology ?

Giuseppe Larosa
Hall of Fame
Hall of Fame

Hello Mac,

can you provide the appropriate

show ip mroute G

for the

(S,G) pair of interest on the routers including last hop router and penultimate hop router ?

the setting of the RP bit on the PIM prune message is one thing that happens as soon as the first multicast packet is delivered over the shared tree  Once the last hop router knows the source S address it performs an RPF check on the S address and if the RPF PIM neighbor for S address is different from RPF neighbor for the RP address the switchover to source based tree is started.

Be aware that you will see two entries

(*, G) entry for the shared tree

(S, G) entry for the source based tree

 

post the whole output so we can check all the possible meaning of flags.

 

Hope to help

Giuseppe

 

 

Hello Giuseppe

 

R8 is my last hop router....sending joins from the Loopback interface:

 

R8#show ip mroute 234.2.2.2

(*, 234.2.2.2), 00:14:55/stopped, RP 11.11.11.11, flags: SJCL
Incoming interface: FastEthernet0/0, RPF nbr 78.78.78.7
Outgoing interface list:
FastEthernet1/0, Forward/Sparse, 00:12:49/00:02:17
Loopback8, Forward/Sparse, 00:14:51/00:00:08

 

(192.168.1.2, 234.2.2.2), 00:11:28/00:01:18, flags: LJT
Incoming interface: FastEthernet0/0, RPF nbr 78.78.78.7
Outgoing interface list:
Loopback8, Forward/Sparse, 00:11:28/00:00:31
FastEthernet1/0, Forward/Sparse, 00:11:28/00:02:17

 

 

R6 and R7 are the PHRs , with R6 having the best route to the RP, while R7 , having the best route to the source...

As you can see, R7 has flagged T for the (S,G) entry, meaning R8 has swithover to the source via R7.

My expectation is that, R6 , should received a prune message from R8. But it seems it didn't. The flag remains at "R" and I believe MC traffic is flowing in R6 towards R8 as it is not pruned and OIL for the S,G entry is not Null....

 

R6#show ip mroute


(*, 234.2.2.2), 00:10:49/00:03:27, RP 11.11.11.11, flags: S
Incoming interface: FastEthernet2/0, RPF nbr 46.46.46.4
Outgoing interface list:
FastEthernet2/1, Forward/Sparse, 00:10:49/00:03:27

 

(192.168.1.2, 234.2.2.2), 00:08:35/00:02:57, flags: R
Incoming interface: FastEthernet2/0, RPF nbr 46.46.46.4
Outgoing interface list: FastEthernet1/0, Forward/Sparse, 00:01:01/00:03:16

 

 


R7#show ip mroute 234.2.2.2

(*, 234.2.2.2), 00:13:17/00:03:21, RP 11.11.11.11, flags: S
Incoming interface: FastEthernet2/1, RPF nbr 67.67.67.6
Outgoing interface list:
FastEthernet0/0, Forward/Sparse, 00:13:17/00:03:21

 

(192.168.1.2, 234.2.2.2), 00:11:00/00:02:21, flags: T
Incoming interface: FastEthernet2/0, RPF nbr 57.57.57.5
Outgoing interface list:
FastEthernet0/0, Forward/Sparse, 00:11:00/00:03:21

 

Hi @mac_mac_net83,

Those outputs show that Routing Table on the LHR (R8#) is pointing to R7# to reach both, the RP (11.11.11.11) and the Source of the Multicast traffic (192.168.1.2).

R8#show ip mroute 234.2.2.2

(*, 234.2.2.2), 00:14:55/stopped, RP 11.11.11.11, flags: SJCL
 Incoming interface: FastEthernet0/0, RPF nbr 78.78.78.7
Outgoing interface list:
 FastEthernet1/0, Forward/Sparse, 00:12:49/00:02:17
 Loopback8, Forward/Sparse, 00:14:51/00:00:08


(192.168.1.2, 234.2.2.2), 00:11:28/00:01:18, flags: LJT
 Incoming interface: FastEthernet0/0, RPF nbr 78.78.78.7
Outgoing interface list:
 Loopback8, Forward/Sparse, 00:11:28/00:00:31
 FastEthernet1/0, Forward/Sparse, 00:11:28/00:02:17

Therefore R8# is not sending PIM Prune with RP-bit set to R6#.

Use show ip rpf 11.11.11.11 and show ip route 11.11.11.11 to know the reason.

 

Cheers.

Hello Mark,

as noted by Hector

from the point of view of the last hop router the upstream interface for the RP address 11.11.11.11 and for the source 192.168.1.2 is the same

(*, 234.2.2.2), 00:14:55/stopped, RP 11.11.11.11, flags: SJCL
>>Incoming interface: FastEthernet0/0, RPF nbr 78.78.78.7

(192.168.1.2, 234.2.2.2), 00:11:28/00:01:18, flags: LJT
>>>Incoming interface: FastEthernet0/0, RPF nbr 78.78.78.7

 

so R8 has no need to send an RP bit set PIM Prune message to anyone.

 

R8 has the same RPF neighbor 78.78.78.7 for both so it is attached to likely R7.

 

check with

show ip rpf 11.11.11.11

show ip route 11.11.11.11

 

show ip route 192.168.1.2

show ip rpf 192.168.1.2

 

Edit:

depending on topology R7 can have won a competition with R6 with a PIM assert message for both the RP address and the source address.

Or R7 is providing the best path to both the RP address and the source address.

Or R8 -R6 PIM neighborship is not up in their  dedicated link.

 

Hope to help

Giuseppe

 

 

THanks everyone! Looks like my routing is messed up. Thanks for the hints and comments. Appreciate it.

Hello


@mac_mac_net83 wrote:

R8#show ip mroute 234.2.2.2

(*, 234.2.2.2), 00:14:55/stopped, RP 11.11.11.11, flags: SJCL
Incoming interface: FastEthernet0/0, RPF nbr 78.78.78.7
Outgoing interface list:
FastEthernet1/0, Forward/Sparse, 00:12:49/00:02:17
Loopback8, Forward/Sparse, 00:14:51/00:00:08

 

(192.168.1.2, 234.2.2.2), 00:11:28/00:01:18, flags: LJT
Incoming interface: FastEthernet0/0, RPF nbr 78.78.78.7
Outgoing interface list:
Loopback8, Forward/Sparse, 00:11:28/00:00:31
FastEthernet1/0, Forward/Sparse, 00:11:28/00:02:17

 

As you can see, R7 has flagged T for the (S,G) entry, meaning R8 has swithover to the source via R7.



Is the SPT path the same as the RP due to incomplete route path ?

 

 

 


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Paul
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